The invention concerns a superconducting magnet configuration for a magnetic resonance apparatus, comprising:                a substantially cylindrical magnet coil with a magnet winding of superconducting wire for generating a magnetic field B in a working volume;        a room temperature bore which is coaxial to the magnet coil and contains the working volume; and        several electrically conducting oscillation systems (R1, R2, R2′) which are disposed radially inside the magnet winding of the magnet coil, wherein each oscillation system (R1, R2, R2′) has a uniform oscillation behavior and can oscillate relative to any other oscillation system, wherein each oscillation system (R1, R2, R2′) is substantially tubular and disposed coaxially to the room temperature bore, wherein each oscillation system (R1, R2, R2′) has an electrical conductivity value p=σ*d with p>1*102 1/Ohm at room temperature, with σ: electric conductivity of the oscillation system and d: minimum wall thickness of the oscillation system in the radial direction,        
wherein each oscillation system (R1, R2, R2′) has a characteristic mechanical value q=E/ρ, with E: average modulus of elasticity of the oscillation system and ρ: average density of the oscillation system,                wherein a low temperature oscillation system (R1) or several low temperature oscillation systems are provided which have temperatures of T1<10K during operation,        and wherein a warm oscillation system (R2) or several warm oscillation systems (R2, R2′) are provided which have temperatures of T2>10K during operation.        
A superconducting magnet configuration of this type is disclosed in DE 101 27 822 A1.
Superconducting magnet configurations are used to generate strong magnetic fields in a working volume. Strong magnetic fields are required, in particular, in nuclear magnetic resonance (NMR) spectroscopy and NMR tomography (MRT) to perform high-quality measurements.
The strong magnetic fields are thereby generated by a magnet coil which is wound from superconducting wire. The magnet coil typically comprises several sections and portions of partially different superconducting materials. The superconducting materials used in the magnet coil are cooled below the transition temperature such that large electric currents and magnetic field strengths can be produced by the magnet coil in this superconducting state. It is cooled e.g. by surrounding liquid helium or by refrigerators which are in thermal contact with the magnet coil.
In NMR experiments, additional fields are added to the strong magnetic field of the magnet coil. These so-called gradient fields are used to encode signals (e.g. local encoding) and are generated by a gradient coil. The gradient coil is usually positioned in the room temperature bore of the superconducting magnet configuration close to the working volume. The current in a gradient coil is typically switched many hundreds of times per second.
Switching of the gradient coil and the associated magnetic field change induce electric currents in nearby electrically conducting, in particular, metallic bodies. When these bodies are simultaneously exposed to a strong magnetic field, which prevails radially inside the magnet winding of the magnet coil, these electric currents produce Lorentz forces which elastically deform the bodies. Due to repeated switching of the gradient coil, the electrically conducting bodies are caused to oscillate.
The oscillating motion of an electrically conducting body in a strong magnetic field, in turn, produces eddy currents in this body. These eddy currents produce heat due to the ohmic resistance in the body and also cause a magnetic field change in the surroundings which may, in turn, produce differing induced currents in neighboring electrically conducting bodies etc. In consequence thereof, all electrically conducting bodies within the magnet winding substantially oscillate with electromagnetic coupling and are heated.
A coil carrier of the magnet coil or an inner wall of a helium tank which contains the magnet coil are e.g. oscillating bodies in a superconducting magnet coil system, in which eddy currents can produce heat. For magnet coils which are in contact with liquid helium, this results in undesired evaporation of large amounts of expensive liquid helium. Moreover, the heat may be transferred to the superconducting wire of the magnet coil, and the superconducting wire could be heated beyond the transition temperature thereby bringing the magnet coil into the undesired, normally conducting state with associated complete decay of the magnetic field as well as strong heating. The magnet coil would then have to be recooled with liquid helium, which is expensive, and the magnetic resonance apparatus would be inoperative for several days.
Conventional gradient coils are actively shielded to reduce the eddy current effects and resulting heating of the magnet coil.
DE 101 27 822 A1 proposes providing a magnet system with an inner, a central, and an outer unit, wherein the units are nested within each other. The mechanical properties of the units are thereby matched, such that the central unit damps transfer of oscillations due to magnetic coupling from the outer to the inner unit. This also reduces heating of the magnet coil.
In a magnetic resonance apparatus comprising a magnet configuration with room temperature bores having large diameters of 40 cm and more, the heat input into the magnet coil is excessively large for permanent operation despite the conventional measures. For cooling the magnet coil with liquid helium, the overall typical supply of liquid helium would e.g. evaporate within a few hours.
In contrast thereto, it is the underlying purpose of the invention to provide a superconducting magnet configuration which further reduces the heat input into the magnet coil.